A static IP address is an IP address that was manually configured for a device, versus one that was assigned by a DHCP server. It's called static because it doesn't change. It's the exact opposite of a dynamic IP address, which does change.
A static IP address is useful if you host a website from home, have a file server in your network, use networked printers, forward ports to a specific device, run a print server, or use a remote access program. Because a static IP address never changes, other devices always know exactly how to contact a device that uses one.
You can add a static IP address to your Metronet services for $10 per month.